I bought a 120mm intake fan which came with a molex connector.
It's very loud. If I use a molex -> 3 pin mb connector, can I slow it down with speedfan?
I have a bunch of unused motherboard chassis fan ports.

You can't slow down your fan with the molex adapter on your power supply. Try connecting it to your motherboard fan ports and control it with speedfan. Alternatively, you can get a fan controller and your problem will be solved.

You can't slow down your fan with the molex adapter on your power supply. Try connecting it to your motherboard fan ports and control it with speedfan. Alternatively, you can get a fan controller and your problem will be solved.

Um, yes you can. Not only does the link posted before you show it but its also one of the easiest ways to slow down a fan.
I usually reference this guide.Get 12V, 7V or 5V for your Fans | silentpcreview.com
Much better explanation. Switch isn't necessary. All you need is a thin flat tipped object to flatten out the molex tip when taking them out and to bend the flanges back out when plugging the wires back into the molex plug. Just make sure the wires are secure in the molex plug